Atomic Layer deposition (ALD) process is a layer-by-layer deposition process comprising alternative exposure and purge steps, where the precursors react with the sample surface in a sequentially one-at-a-time manner. A typical ALD process may include the following steps: 1) place a sample in a sealed chamber, evacuate the chamber with a vacuum pump, and keep the sample at certain temperature; 2) introduce the first precursor, say, precursor A, into the chamber. Precursor A may or may not be carried by an inert gas that is called “carrier gas”. In this step, precursor A will chemically react with the sample surface, forming a chemisorbed layer of molecules of precursor A on the sample surface. This step is usually called as “precursor-exposure” step; 3) pump or purge the chamber so as to remove un-reacted precursor A and reaction byproducts, leaving the chemisorbed layer of molecules of precursor A on the sample surface. This step is usually called as “purge” step; 4) introduce the second precursor, say, precursor B, into the chamber, and precursor B will chemically react with the sample surface, converting the chemisorbed layer of molecule A into a solid deposition. Again, this step is called as “precursor-exposure” step; 5) pump or purge the chamber so as to remove un-reacted precursor B and byproducts, providing a fresh surface for another layer of chemisorptions of precursor A. Again, this step is called “purge” step; 6) repeating step 2 to 5 to achieve a number of layers as needed, so as to obtain the thickness of the coating as needed. Detailed ALD process may vary, but they all comprise alternating “precursor-exposure” step and “purge” step, and materials are deposited on a surface in a layer-by-layer manner.
Using an ALD process, the physical or/and chemical properties of a sample surface can be modified, and coatings can build up in a one-atomic-layer by one-atomic-layer fashion, with thickness control in atomic-level precision. In addition, the coatings or modifications are usually uniform and conformal throughout the whole sample surface because the surface reaction and surface adsorption are usually uniform and conformal.
Typically, an ALD apparatus comprises a deposition chamber with at least one pumping port so as to remove gas from the chamber, and at least one gas-injection port so as to bring gas into the chamber.
In general, a successful ALD process requires a good precursor-exposure step and a purge step, which means that precursor molecules should be able to freely reach the sample surface; and after the reaction of precursors at the sample surface, the reaction byproduct and un-reacted residual precursors should be able to be removed easily from the sample surface vicinity. In addition, during the steps of precursor introduction and gas removal, the sample should stay inside of the deposition chamber instead of being blown away by the gas flow.
For a wafer sample, the above requirements can be easily satisfied because the surface geometry of a wafer is simply flat. Transport of gas molecules to and away from this flat surface is easy. In addition, the wafer samples are usually heavy enough so that the samples won't be blown away by the gas flow.
However, when running ALD on small particles, or powdered materials, there are at least three issues: 1) for a plurality of powders, the powders buried at the bottom have less chance to be exposed to the reactant gases, causing problem for “precursor-exposure” step; 2) the gas molecules trapped in between of powders at the bottom have a less chance to be pumped or purged away, causing problem for “purge” step, resulting in non-ALD depositon in these locations; 3) the powders are light and can be easily blown away by the gas flow during the steps of introducing precursors for “precursor-exposure” and the step of pumping for “purge”, making powders being carried by gas flow thereby sticking on the chamber walls, or entering the pumping port as mentioned in paragraph [0005].
Despite of above issues, there is an increasing interest in doing ALD on powdered samples (including small fibers) as ALD can be used to modify the surface property of the powdered samples, or to make a thin layer of catalytic or other functional materials on the powder surface. For example, ALD of platinum on porous carbon powders is of great interest in fuel cells applications; ALL) of oxide on porous powders can be used for battery electrodes, or super capacitors; ALD of photocatalytic TiO2 on porous powders can be used for water or air detoxification; ALD of catalysts on ceramics powders can be used to remove NO, CO etc from auto exhaust or the flue gas from electrical plants etc.
To facilitate ALD on powder samples, some researchers have developed a fluidized-bed ALD system, where the powders are blown up and dispersed by turbulent gas flow so that all the powders can have good chances to be exposed to the reactant gases. But there are several disadvantages in this approach: 1) the ALD chamber has to be relatively large to satisfy the configuration of a “fluidized bed”, e.g. several feet in height, especially when less reactive precursors are used and longer residence time is needed to complete the reaction; 2) fluidized bed also requires relatively high gas pressure and large gas flow so that the particles can be blown up by the gas flows which causes a big waste of carrier gases; 3) powders will fly around vigorously inside the chamber, and a porous filter with small pores has to be used to prevent powders from being pumped away, but this porous filter will trap powders no that many powders are wasted by being trapped inside the pores. Further, once the powders are trapped inside the pores, the filter will be blocked. As a consequence, the filter has to be cleaned or replaced frequently; 4) since powders will fly around inside the chamber, many powders will stick on the chamber walls or be trapped in the porous filters, which are hard to collect after ALD, causing waste of powders and contamination for the following ALD process. Therefore, for this fluidized-bed ALD system, it is hard to process a very small amount of powder due to loss of powders in the filter and the chamber walls. It doesn't work very well if research-scale small amount of powdered samples are processed. [Ferguson et al, Powder Technol, No. 156, page 154, 2005]
In another known art, an ALD system with rotary cylinder is used for powder ALD [McCormick et al, J. of Vac. Sci. and Technol. A, January/February 2007, p 67]. In this art, the ALD chamber is a rotary cylinder, which rotates along a horizontal axis to agitate the powders so as to help achieving good results in “precursor-exposure” step and the “purge” step. In this known art, there are two distinctive features: 1) the cylinder rotates along a horizontal axis; 2) a porous filter has to be used to prevent powders from being blown away. Again, the porous filter will trap powders and may be blocked by the powders after long-term usage.
Therefore, there is a need of developing improved ALD system to process powdered samples, wherein: 1) powders are agitated so that powders buried in the bottom of the pile can be constantly brought to the top of the pile, enabling all powders to have a good chance to be exposed to reactant or purge gases during the “precursor-exposure” and “purge” steps; 2) powders won't be blown away by gas flow, and powders are confined within the ALD chamber without using porous filters.